tcp连接在断网后的恢复能力
- 2023-08-05 05:05:57
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做项目中遇到一个问题。两台机器上用socket建立一个TCP连接,双向通信,流量很大,这时,通过在路由器上设置100%的丢包率将网络断开,这时 socket当然是发不了包,也收不了,出现大量的重传,然后,取消路由器上的设置,恢复网络,结果,TCP连接client去往server的流量正常了,但server去往client却不通,任凭你如何使劲的send,返回值就是0,而且errno为EAGAIN。
我用tcpdump看了一下此时的包数据(tc2是server,tc1是client):
12:08:21.020291 IP tc1.corp.com.42171 > tc2.corp.com.3003: S 4009389430:4009389430(0) win 5840
12:08:21.020571 IP tc2.corp.com.3003 > tc1.corp.com.42171: R 0:0(0) ack 4009389431 win 0
12:08:38.934329 IP tc2.corp.com.3903 > tc1.corp.com.3904: P 2398055392:2398056153(761) ack 2538876742 win 724
12:08:38.934519 IP tc1.corp.com.3904 > tc2.corp.com.3903: . ack 2165 win 13756
12:08:39.958457 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1:763(762) ack 2165 win 13756
12:08:39.958485 IP tc2.corp.com.3903 > tc1.corp.com.3904: . ack 763 win 1448
12:08:39.958653 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 763:881(118) ack 2165 win 13756
12:08:39.958660 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 881:997(116) ack 2165 win 13756
12:08:39.958719 IP tc2.corp.com.3903 > tc1.corp.com.3904: . ack 997 win 1448
12:08:39.958890 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 997:1114(117) ack 2165 win 13756
12:08:39.958898 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1114:1232(118) ack 2165 win 13756
12:08:39.958903 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1232:1349(117) ack 2165 win 13756
12:08:39.958971 IP tc2.corp.com.3903 > tc1.corp.com.3904: . ack 1349 win 1448
12:08:39.959141 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1349:1466(117) ack 2165 win 13756
12:08:39.959149 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1466:1583(117) ack 2165 win 13756
12:08:39.959154 IP tc1.corp.com.3904 > tc2.corp.com.3903: P 1583:1700(117) ack 2165 win 13756
12:08:39.959222 IP tc2.corp.com.3903 > tc1.corp.com.3904: . ack 1700 win 1448
tc2不发自己的数据,却只是一味的ACK从tc1传来的数据,等上半个小时,依然如此。它为什么不发呢?
最后发现是因为我们在socket上设了TCP_NODELAY。去掉这个设置,重启程序,断网恢复以后,TCP双向正常工作。同样用tcpdump看:
16:05:38.782427 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: P 0:887(887) ack 1 win 26064
16:05:38.782619 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 3783 win 25352
16:05:38.782634 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 3783:5231(1448) ack 1 win 26064
16:05:38.782637 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 5231:6679(1448) ack 1 win 26064
16:05:38.782890 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 5231 win 25352
16:05:38.782896 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 6679:8127(1448) ack 1 win 26064
16:05:38.782898 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 8127:9575(1448) ack 1 win 26064
16:05:38.782901 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 6679 win 25352
16:05:38.782904 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 9575:11023(1448) ack 1 win 26064
16:05:38.783183 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 8127 win 25352
16:05:38.783188 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 11023:12471(1448) ack 1 win 26064
16:05:38.783191 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 9575 win 25352
16:05:38.783193 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 12471:13919(1448) ack 1 win 26064
16:05:38.783196 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 11023 win 25352
16:05:38.783199 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 13919:15367(1448) ack 1 win 26064
16:05:38.783201 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 15367:16815(1448) ack 1 win 26064
16:05:38.783502 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 12471 win 25352
16:05:38.783506 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 16815:18263(1448) ack 1 win 26064
16:05:38.783509 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 13919 win 25352
16:05:38.783512 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 18263:19711(1448) ack 1 win 26064
16:05:38.783514 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 15367 win 25352
16:05:38.783517 IP tc2.corp.alimama.com.3903 > tc1.corp.alimama.com.3904: . 19711:21159(1448) ack 1 win 26064
16:05:38.783519 IP tc1.corp.alimama.com.3904 > tc2.corp.alimama.com.3903: . ack 16815 win 25352
tc2这次发自己的数据流了,tc1对其ACK,过了一段时间,tc1也开始发数据,最后双向正常。
为什么带了TCP_NODEALY的socket,在网络好了以后恢复不了正常?
看看recv系统调用的实现(2.6.9内核),一直追溯到tcp_recvmsg函数:
[net/ipv4/tcp.c --> tcp_recvmsg]
813 while (--iovlen >= 0) {
814 int seglen = iov->iov_len;
815 unsigned char __user *from = iov->iov_base;
816
817 iov++;
818
819 while (seglen > 0) {
820 int copy;
821
822 skb = sk->sk_write_queue.prev;
823
824 if (!sk->sk_send_head ||
825 (copy = mss_now - skb->len) <= 0) {
826
#p#副标题#e#
827 new_segment:
828 /* Allocate new segment. If the interface is SG,
829 * allocate skb fitting to single page.
830 */
831 if (!sk_stream_memory_free(sk))
832 goto wait_for_sndbuf;
833
834 skb = sk_stream_alloc_pskb(sk, select_size(sk, tp),
835 0, sk->sk_allocation);
836 if (!skb)
837 goto wait_for_memory;
831行判断sndbuf里还有没有空间,如果没有,跳到wait_for_sndbuf
[n
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